The file of this patent contains at least one drawing executed in color. Copies of this patent with color drawings will be provided by the Patent and Trademark Office upon request and payment of the necessary fee.
The invention relates to a method for detecting processed gem-quality colorless and fancy-colored diamonds. In particular, the invention relates to a method for detecting processed diamonds that have been produced from inferior-grade discolored diamonds by a high-temperature and high-pressure (HPHT) process.
Diamonds are conventionally divided into four main categories, which are designated as Type Ia, Type Ib, Type IIa, and Type IIb. In reality, there is a smooth change in impurity concentration/arrangement between the four types so that intermediate varieties thereof also exist.
Type I diamonds contain nitrogen as the major impurity, and can be divided into Type Ia diamonds and Type Ib diamonds. Type Ia diamonds contain a nitrogen impurity that exists in an agglomerated state. The agglomerated state exists as nitrogen pairs, called xe2x80x9cA Centersxe2x80x9d (Type IaA), nitrogen clusters comprising four nitrogen atoms called xe2x80x9cB centersxe2x80x9d (Type IaB), and mixtures thereof (Type IaA/B). Type Ib diamonds contain nitrogen as isolated single nitrogen atoms called xe2x80x9cC Centersxe2x80x9d. Some Type I diamonds may also contain clusters of three nitrogen atoms called xe2x80x9cN3 Centersxe2x80x9d.
Type Ia diamonds comprise over about 98% of the larger clear natural diamonds. Type Ib diamonds are rarer and amount to only about 0.8% of natural diamonds. Type Ia diamonds may also contain platelets, which are small flat inclusions that a few atoms thick and about 300 atoms across. The platelets may contain some nitrogen in an unspecified form. Type Ia diamonds also may contain voidites, which are small equiaxed cavities that are either vacant or that contain nitrogen in an unspecified form. Voidites tend to typically exist in Type IaA/B diamonds or Type IaB diamonds.
Natural diamonds may possess a color that can range from clear and colorless diamonds to yellow, orange, red, pink, blue, brown, and even green colored diamonds. Intermediate colored diamonds are also possible. Diamonds can even appear to change color depending on the lighting conditions. These diamonds are known in the art as xe2x80x9cchameleonxe2x80x9d diamonds. For natural diamonds, a brownish color is the most common, and may occur in up to about 98% of mined natural diamonds. The brownish and pinkish color is believed to be a result of plastic deformation of the diamonds after they were formed.
Most natural Type Ia diamonds have a brownish color. A brownish color may result from a mixture of many other colors. For example, the brownish color may result from a mixture of yellowish coloring (such as from isolated nitrogen atoms (C Centers) or N3 centers) with some blackish coloring (such as from submicroscopic inclusions of graphite). The mixture of yellowish and blackish colorings will produce a brownish color. Further, a brownish coloring in a diamond can be formed from a mixture of color centers that produce a greenish coloring in a diamond with a color center that produces a reddish coloring in a diamond. An infinite number of color combinations that produce a brownish color is possible. Therefore, it is generally impossible to determine the color centers causing the color of a diamond by its color alone.
Type II diamonds are conventionally defined as diamonds that contain less than 1 PPM of nitrogen. The selection of 1 PPM nitrogen level is historically related to the fact that this was the level of nitrogen routinely detectable in infrared spectrums of gem diamonds. Type II diamonds are further divided into Type IIa and Type IIb. Type IIa diamonds have no other impurities other than nitrogen at less than a 1 PPM level. Type II diamonds contain boron in the parts per million range and the boron concentration always exceed any nitrogen concentration in the crystal. Type IIb diamonds are blue in color and are extremely rare, and thus have a high value per carat as jewelry items.
The pricing of diamonds typically is a function of their color. Diamonds that exhibit fancy colors, such as the canary yellows, blues, reds, pinks, and greens, are rare and tend to have the highest prices. Diamonds that are classified as xe2x80x9ccolorless diamondsxe2x80x9d command the highest prices after fancy colored diamonds. The degree of colorlessness lends to a nonlinear price effect of the diamond. Even the faintest tinge of yellow can considerably reduce the price of colorless diamonds. Brownish diamonds are an exception to the above-stated fancy color diamond market, as they are very common. Brownish diamonds typically have been culled and used as industrial diamonds, and thus the brownish diamonds are relatively inexpensive.
An aesthetic and economic incentive thus exists to change relatively inexpensive brownish colored diamonds to either of the more valuable colorless diamonds or to fancy color diamonds in view of the relative scarcity and beauty of fancy colors, colorless, and the commonality of brownish diamonds. Various methods have been proposed and used to treat diamonds. For example, irradiation has been used to change the diamond color from typically unattractive off-colors to attractive blue, green, orange, black, and yellow colors. Also, electrons, neutrons, gamma rays, and alpha particles have been proposed and used to produce irradiation-produced color centers in diamonds. Neutron, gamma, and electron irradiation have been generally used because these methods produce a more uniform coloration in diamonds due to an effective penetrating power into the diamonds. However, neutrons used for treating diamonds may introduce dangerous side effects since radioactive species can be produced in diamond inclusions by neutron activation. Additionally, typical electron or alpha irradiation methods merely develop a superficial color that is confined to outer portions of the diamonds.
Further, methods such as, but not limited to, laser-drilling, fracture-filling, and surface-coating have been proposed and used to treat diamonds in attempts to increase their value. These methods, while somewhat effective in the color change, are easily detected by conventional gemological practices. For example, an observation of diamonds in an optical microscope may reveal treating by laser-drilling, fracture-filling, and surface-coating methods.
Most diamond treating is easily detected by looking at infrared and optical spectrums of the diamonds. The treating produces infrared and optical spectrum characteristics (also known as xe2x80x9cdetection signaturesxe2x80x9d), which include detectable different radiations that are produced by vacancies in diamonds. The detection signatures or optical spectra signature characteristics are observed in the GR1 band of the visible spectrum; and absorption at 740.9 nm and from 412-430 nm by the GR1 band that produces a green, blue-green, dark green, or even a black color in the diamond by absorption.
Vacancy color centers introduced by irradiation may be modified by high-temperature annealing treatments to produce diamond colors that range from blue to pinkish to red to green colors. Annealing can be conducted at temperatures as low as about 600xc2x0 C., because the large number of vacancies introduced by irradiation temporarily increases the mobility of nitrogen and other impurities in the diamonds. During the annealing, the vacancies diffuse to, and are absorbed by at least one of vacancy sinks in the diamond, such as, but not limited to, free surfaces, dislocations, and inclusion interfaces, and complexes of nitrogen, such as A, B, and C Centers in the diamonds. As the vacancies disappear, their immediate influence on the observed diamond color lessens. Thus, the diamond color gradually changes from blue to green to brownish to yellow and back to the original color of the diamond. The annealing can be stopped at any point during the annealing to produce a desired treated diamond color. Multiple irradiation steps and annealing steps may be conducted to manipulate and change the treated diamond color.
Recently, attempts to treat diamonds by annealing them at progressively higher temperatures to eliminate indications of irradiation, for example optical spectra signature characteristics. For example, the GR1 line that results from a vacancy formed during irradiation treatment begins to disappear above 400xc2x0 C. as the vacancies anneal out of the diamond crystal. Other irradiation lines, however, persist to higher temperatures. Conventional color change treatments for diamonds may produce color changes therein; however the material and mechanical properties and characteristics of the diamonds are often impaired. The elimination of any indications of irradiation in diamonds is desired because xe2x80x9ctreatedxe2x80x9d diamonds have a discounted value with respect to natural diamonds.
Electron or neutron irradiation of Type Ia diamonds and a subsequent heat treatment generates H3 (Nitrogen-Vacancy-Nitrogen) centers and H4 (Nitrogen-Nitrogen-Vacancy-Nitrogen-Nitrogen) centers therein. These centers provide an amber gold color to the treated diamonds. The H3 centers and H4 Centers, respectively, have absorption bands at 503 nm and 496 nm, which are characteristic optical spectra signatures of these treated Type Ia diamonds. The ratios of H3 centers to H4 centers and the ratio of A centers to B centers in the diamonds can distinguish natural diamonds from these irradiation and annealed treated diamonds.
Further, a vacancy in a diamond can combine with a single nitrogen C Center and form an H2 Center (Nitrogen-Vacancy). The H2 Center can impart a distinguishable optical spectra signature characteristic of treated diamonds. The H2 Center can cause a greenish color in the diamonds and its optical spectra signature characteristic is a vibronic absorption band at 637.3 nm. By absorbing light in the red at 637 nm, the remaining light coming from these treated diamonds is shifted towards a greenish color, and thus a detectable spectra signature characteristic for these treated diamonds is evident and apparent.
Recently, processing has improved a diamond""s color, such as by exposing the diamonds to high-pressure high-temperature (HPHT) conditions, which attempt to simulate those conditions in the earth""s mantle. Many of these color-improved diamonds are Type I diamonds with low nitrogen concentrations or Type II diamonds with nominally no nitrogen. Conventional analyzing of these color-improved diamonds has not been able to discern that they have been processed. It is believed that the analyzing fails because of: the low concentration or lack of nitrogen; measurements of A, B, and C Center concentrations, measurements of H2, H3 and H4 center concentrations, measurements of optical and infrared spectral signatures associated with nitrogen, nitrogen complexes, vacancies and vacancy-nitrogen pairs, or vacancy-nitrogen-complex agglomerates are not able to determine whether the diamond has been exposed to HPHT processing.
Therefore, a method for determining the existence of a processed diamonds is needed.
The invention sets forth a method for detecting whether a natural diamond has been processed at high pressure and high temperature (HPHT) conditions. The method comprises steps of disposing the diamond in a cryostat that is provided at temperatures equal to or less than liquid nitrogen; illuminating the diamond with a laser beam; recording an optical spectrum of the diamond with a photoluminescence spectrometer; and examining the optical spectrum of the diamond to detect an absence of selected photoluminescent spectral lines.
The invention also sets forth a method for predicting whether a natural diamond has been processed at high pressure and high temperature (HPHT) conditions. The prediction method comprises steps of disposing a diamond in a cryostat that is provided at temperatures equal to or less than liquid nitrogen; illuminating the diamond with a laser beam; recording an optical spectrum of the diamond with a photoluminescence spectrometer; examining the optical spectrum of the diamond to detect an absence of selected photoluminescent spectral lines; and predicting that the diamond has been treated if at least one of selected photoluminescent spectral lines is not in the optical spectrum.
Further, the invention also sets forth a method for detecting whether a natural diamond has been processed at high pressure and high temperature (HPHT) conditions. This method comprises steps of disposing the diamond in a cryostat that is provided at temperatures equal to or less than liquid nitrogen; illuminating the diamond with a laser beam; recording an optical spectrum of the diamond with a photoluminescence spectrometer; and examining the optical spectrum of the diamond to detect an absence of a spectral line at 2.53 eV. If the 2.53 eV spectral line is not present in the optical spectrum, the method determines with up to about a 95% probability exists that the diamond was processed under HPHT conditions, and if the 2.53 eV spectral line is present, the method determines that the diamond has not been subjected to HPHT conditions.
Another aspect of the invention, sets forth a method for predicting whether a natural diamond has been processed at high pressure and high temperature (HPHT) conditions. This prediction method comprises steps of disposing a diamond in a cyrostat that is provided at temperatures equal to or less than liquid nitrogen; illuminating the diamond with a laser beam; recording an optical spectrum of the diamond with a photoluminescence spectrometer; examining the optical spectrum of the diamond to detect an absence of spectral lines at 2.53 eV; and predicting that the diamond has been treated if at least one of selected photoluminescent spectral lines is not in the optical spectrum. If the 2.53 eV spectral line is not present in the optical spectrum, the prediction method determines with up to about a 95% probability exists that the diamond was processed under HPHT conditions, and if the 2.53 eV spectral line is present, the prediction method determines that the diamond has not been subjected to HPHT conditions.
These and other aspects, advantages and salient features of the invention will become apparent from the following detailed description, which, when taken in conjunction with the annexed drawings, where like parts are designated by like reference characters throughout the drawings, disclose embodiments of the invention.